Originating from high-end smart phone fields, the multi-point touch (or multi-touch) technology is now spreading to the entire customer electronics industry, including mid-end and low-end mobile phones, game consoles and systems, media players, navigation systems, e-readers, tablet computers, and the like.
Capacitive touch screens operate with charge transfer through human body. A capacitive touch screen is typically a four-layer compound glass screen. The three inner layers are composed of two transparent conductive films, such as indium-tin-oxide (ITO), and an insulating layer there between. The outermost layer of the four-layer compound is a thin layer of silica glass protection layer. The inner and outer ITO films are etched to have bar-shaped electrodes along two perpendicular axes, thereby forming meshes (sensing capacitors). When a finger touches a metal layer, a coupling capacitor is formed on the surface of the touch screen where the finger touches. For high-frequency currents, the capacitor acts as a direct conductor. Since the human body has a body-to-ground capacitance, the finger absorbs a small amount of currents from the touch point. The position of the touch point can therefore be detected by monitoring the changes of the currents of the meshes.
The core component of the projected capacitive touch screen is the glass coated with ITO films on both sides. The inner and outer ITO films of the projected capacitive touch screen do not cover the entire screen, but are made into horizontal and vertical bar patterns or diamond patterns that are staggered between the inner and outer films.
FIG. 1 illustrates a single layer electrode and a conducting circuit disclosed in Chinese Patent Application CN102033672A. It comprises a substrate 9, an electrode 10 and a conducting circuit 11. The electrode 10 is composed of multiple comb-shaped structures, in which each two adjacent comb-shaped structures intersect with each other to form the electrode. The conducting circuits 11 are vertical or horizontal lines, and are connected to the respective electrodes. One issue with such design is that, after the substrate and the LCD panel are joined, noticeable rainbow effects may appear. Such rainbow effects have adverse impact on transparency, linearity, and display effects, resulting in unstable performance of the touch screen.
Using two ITO films obviously increases both the complexity and the costs in the fabrication process, and reduces the manufacture yield.
Therefore, using a single ITO film is preferable. However, bridging among electrodes is frequently required in the fabrication process with a single ITO film. Due to differences in thickness between the insulating film and the ITO or metal film, bridges may crack. After the capacitive touch screen is affixed to the LCD, using horizontal and/or vertical straight line wire may cause color stripes, similar to the rainbow effect. This creates discomfort to human eyes and affects the image display effects.